In U.S. Pat. No. 3,510,393 assigned to the assignee of the instant application, a hollow glass fiber strand article is described. The hollow glass fibers of the claimed strands typified in this patent are fibers having outside diameters of 0.0003 to 0.003 inches and having 10 to 65 percent of their volume hollow. The hollow glass fibers described in the aforementioned patent found use as a resin reinforcement in filament wound applications such as radar domes, and may be used as a resin reinforcement for motor cases, storage tanks and the like. The advantage of the hollow glass fibers was to provide a significant improvement in the strength to weight ratio of reinforcing fibers utilized to reinforce resin fiber composites. Also provided, as stated in the aforementioned patent, was a significant reduction in the dielectric constant of materials reinforced with the hollow glass fibers over those reinforced with solid fibers. In U.S. Pat. No. 3,268,313, apparatus suitable for use in manufacturing the hollow glass fibers of the aforementioned U.S. Pat. No. 3,510,393 patent is shown. Similarly, in U.S. Pat. No. 3,421,873, alternative apparatus and methods are described where fibers can be produced which have intermittent hollowness provided along their length.
While the aforementioned apparatus, methods and fibers have found some utility in the marketplace their use has been limited due to several factors. One factor was the high cost of preparing the hollow glass fibers. This cost was high due to the fact that the apparatus was difficult to operate on a continuous basis without many process interruptions occurring during the formation of the fibers. The manufacturing costs consequently required high selling prices which was a deterrent in the marketplace. Further, the hollow fibers containing strands produced by the processes and apparatus described in these patents while containing a substantial number of hollow fibers when initially produced from commercial bushings were found after a period of time to contain significant numbers of solid fibers as well as the hollow glass fibers. Still further, the hollow glass fibers produced in a multiple fiber strand had little or no uniformity with respect to the concentricity of the central lumen of the fibers. Stated another way, the K value of the fibers, i.e., the ratio of the inside diameter of the lumen to the outside diameter of the fibers was found to be very erratic and subject to wide variations in a given strand of fibers.
Thus, despite the prior art a need still exists for processes and apparatus for producing hollow glass fibers of more uniform configuration. Still further a need exists for hollow glass fiber strands containing hollow filaments or fibers therein which possess more uniform ratios of inside to outside internal diameters and better concentricity of the central lumen of the fibers and for apparatus for producing such fibers.
In accordance with the instant invention improved hollow glass fibers are provided by using the Applicants' improved bushing assembly. The bushing of the instant invention is characterized by providing a central lumen in each of the fibers it produces having good concentricity and a more uniform K value than fibers heretofore produced. The fibers produced also have a more uniform strength to weight ratio than those previously made by prior art bushings.